Change of tax class is necessary when a person's living status is changed in Germany.
Some of the cases are like marriage, spouse working , second job etc.. and there are 6 tax classes available.

First of all , a tax card (Lohnsteuerkarte) is necessary for people to start working in Germany and is provided by the local authorities and has to be handed over to employer. The local authorities based on the person's status will issue a tax card with category I, II, III ..VI. Employers pay taxes to government based on the tax class of the employee.

Tax class I is always for Singles and is the most taxed class unfortunately.
III for married persons...

To change an existing tax class , one has to show the proof of change in living status. To change a tax class from I to III, one has to show the following documents:

• Proof of Marriage(legalized German translation in case its in a different language)
• Visa for spouse after coming to Germany(Not the 3 months reunion visa issued outside Germany)
• Anmeldung (Registration) of Spouse at the local authorities(Rathaus, Landratsamt or KVR). This is also mandatory for the above
• Tax Card I, for exchange to III

And in the same way , the respective procedures for other changes too, and its always better to call the authorities to know what they need.

Tip:
For newly married , the change of class I to III only can be done when the spouse is registered inside Germany before end of the year and then in the next year one can claim the taxes for the whole year calculated as class III. It is not when one gets married but the date when the spouse joins the family.

Good luck..